Mark Twain referred to it with profundity (5)

I believe the answer is:

depth

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'profundity' is the definition.
(synonyms)

I don't understand how the rest of the clue works.

(Other definitions for depth that I've seen before include "Distance from the surface to the bottom" , "Distance from top down as in water" , "Distance downwards" , "Insight" , "Thoroughness" .)

I've seen this clue in the Evening Standard.

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